Plastic working such as wire drawing, pipe drawing, plate press, heading, and forging requires a lubricating film at the frictional interface between a mold and a material to be worked, and if this lubricating film is insufficient, defects will be caused such as difficulty in working into desired shapes and seizure formation. In particular, cold forging produces an extremely high contact pressure between a mold and a material to be worked, and the mold and the material to be worked relatively slide with the enlarged surface of the material to be worked even to dozens of times. While the lubricating film therebetween requires a high friction reducing ability and a seizure suppressing ability, handling with the use of a lubricating oil is difficult in such an environment, and lubrication with the use of a solid film is thus typically used.
Chemical conversion coating film for crystallization of zinc phosphate crystals on steel surfaces in combination with soap based lubricants, which are commonly known as bonderizing and bonderlube coating films have been long used extensively for lubricating films in the field of cold forging. The zinc phosphate crystals have cleaved facets with a weak bonding force between crystal lattices, and produce cleavages against the shear force at forging frictional interfaces to reduce frictions, and also repair and coat materials to be worked. For this reason, the zinc phosphate crystal films excel in seizure suppressing ability. For the soap based lubricant for coating the zinc phosphate crystal films as an upper layer, alkali soap is typically used which serves to reduce frictions. At the interface between the zinc phosphate crystals and the alkali soap layers, a zinc soap layer that is excellent in lubricating property is also produced by double decomposition reaction to further enhance the lubricating property. The combination of the excellent ability of the phosphate film to resist seizure with the soap lubricating layer associated with a reaction stably supplies lubrication in cold forging. It is not an exaggeration to say that most of the lubricating films used in the current cold forging industry are bonderizing and bonderlube coating films.
On the other hand, with the recently heightened consciousness of environmental conservation, film formation methods for bonderizing and bonderlube coating films have been acknowledged as a problem. In a bonderizing treatment for dissolving and then crystallizing iron and steel materials, there is a need to eliminate iron constantly dissolved into the treatment liquid to the outside of the system as by-products such as iron phosphate crystals. Large amounts of heavy-metal containing waste water, soapy effluent, and the like are discharged, which result in large amounts of industrial waste. In addition, in the treatment process in which the treatment bath temperature reaches even 80° C. or higher, the heat source, the supply of volatilized water, etc. are also fairly costly. In particular, bonderizing treatment equipment directed to steel-wire coiled materials, pipes, and the like has a considerably large scale, and also has high environmental burdens, and there has been thus an urgent need to take countermeasures.
Recently, in order to solve such problems, new environmentally-sound lubricating coating films are being developed which aim at alternatives to bonderizing as exemplified below. Many of these lubricating coating films can be formed by a simple process of just applying a coating treatment liquid to the surfaces of objects and then drying the liquid, and are thus called one-pack type lubricating coating films, and attracting attention.
Patent Literature 1 (Japanese Patent No. 3517522) consists in an aqueous lubricant for cold plastic working containing a specific water-soluble inorganic salt, a solid lubricant, an oil component, and a surfactant in specific proportions. The films formed on the surfaces of iron and steel materials contain each lubricating component based on the water-soluble inorganic salt which has strong adhesion, and introduce the lubricating components to the worked interfaces between the surfaces and a mold. An example with a backward extrusion test as a forging test with a high degree of difficulty in working demonstrates a cold forging performance which is equivalent to comparative bonderizing and bonderlube treatment, and is generally understood as a candidate for alternatives to the bonderizing and bonderlube treatment.
Patent Literature 2 (Japanese Patent No. 3314201) consists in a water-borne cold-forging lubricant of steel or steel alloy characterized in that it is obtained by dispersing an alkylphosphonic acid derivative having a specific structure in water along with a surfactant. In the evaluation of lubricating coating films obtained by forming the lubricant into steel materials, with various types of sliding tests and forging tests, or forging with an actual machine, the films are considered to show favorable results even as compared with bonderizing and bonderlube coating films.
As described above, the lubricating performance of one-pack type lubricating coating films as new lubricating coating films in cold forging is approaching the practical level. FIG. 1 shows line configuration examples of bonderizing treatment and one-pack type lubricating coating treatment. The process of the one-pack type lubricating coating treatment produces no waste water, industrial waste or the like, and requires a small space and a low energy cost for the coating treatment. It is also capable of in-line processes in which the coating treatment unit is directly connected to a forging machine, and has the potential to succeed in significantly improving the layouts of future manufacturing sites.
In recent automobile industry, efforts have been advanced which are aimed at further increasing the efficiency of part manufacturing, and studies have been carried out in which cold forging is intended for even complex-shape parts which have been thus far formed by cutting work. Closed forging with a high degree of difficulty is frequently used for filling even details of a complex mold shape with a material to be worked, and the surface of the material to be processed, which is drawn by working with a large amount of change, is forced to relatively slide with respect to the mold surface under extremely high contact pressure. Lubricating coating films have important roles such as preventing seizure by preventing direct contact between the mold and the material to be worked even while being located at the frictional interface, and reducing the friction for promoting plastic flows of the material to be worked. The lubricating coating films are heavily involved in all of workability of complex shapes, dimensional accuracy, mold life, etc., and the bonderizing and bonderlube coating films and one-pack type lubricating coating films described previously are even being considered inadequate, under the condition that the performance required for the lubricating coating films is becoming more and more stringent.
Disclosures of high-performance lubricating coating films aimed at dealing with severer working, such as in the closed forging field for complex shape parts, include Patent Literature 3 (International Publication No. WO2002/012419). The disclosed aqueous lubricant for plastic working of metal materials contains (A) an water-soluble inorganic salt, (B) one or more lubricating agents selected from molybdenum disulfide and graphite, and (C) a wax, is characterized in that these components are dissolved or dispersed in water, and the solid content concentration ratios (ratios by weight) (B)/(A) and (C)/(A) are respectively 1.0 to 5.0 and 0.1 to 1.0, and raises the performance by containing one or more selected from molybdenum disulfide and graphite contained at the given ratio, as compared with one-pack type lubricating coating films disclosed before that in Patent Literature 4 (Japanese Patent Application Laid-Open No. 2000-63880), etc. These beneficial effects are considered due to friction relaxation by flatting of the so-called solid lubricant, such as molybdenum disulfide and graphite, into thin films over frictional interfaces, and seizure suppression by the surface coating, and believed to suggest importance of the roles of solid lubricants in lubricating coating films intended for forging with a high degree of difficulty.
On the other hand, from recent working environment situations requesting cleaner work environments, the use of black substances has been disliked in many cases, there have been also moves to demand the elimination of industrial raw materials that face risks such as instability of raw material procurement and pricing due to the international situation, and thus, in the future, it will not be possible to rely on lubricating coating films containing black solid lubricating materials such as molybdenum disulfide, tungsten disulfide or graphite. Against such a background, there has been demand for the emergence of a novel solid lubricating material which is less likely to face risks due to raw material procurement or cost fluctuations, and in a non-black color that is less likely to contaminate work environments, and which is able to demonstrate excellent forging performance.
As non-black solid lubricants, melamine cyanurate, boron nitride, carbon fluoride, etc. are famous, and many of lubricants containing these materials are disclosed. Patent Literature 5 (Japanese Patent Application Laid-Open No. HEI 10-36876) as an example thereof discloses an example of a lubricating coating film containing melamine cyanurate, which is supposed to keep a lubricating property equivalent to those of phosphates. However, these solid lubricants are generally high in price, and thus difficult to use, and moreover, in order to stably blend these solid lubricants into lubricating coating films, there is a need to disperse the lubricants over a long period of time while grinding the lubricants into microparticles with the use of an expensive grinding disperser as typified by, for example, beads mills. Therefore, the investment in the grinding disperser and the manufacturing cost from the manufacturing time have been substantially increased, and the solid lubricants are thus not realistic as a technique introduced into “manufacturing sites” currently calling for cost reductions.